首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   3725篇
  免费   66篇
  国内免费   3篇
电工技术   9篇
综合类   2篇
化学工业   302篇
金属工艺   33篇
机械仪表   56篇
建筑科学   88篇
矿业工程   6篇
能源动力   33篇
轻工业   93篇
水利工程   29篇
石油天然气   11篇
无线电   89篇
一般工业技术   302篇
冶金工业   2551篇
原子能技术   11篇
自动化技术   179篇
  2023年   14篇
  2022年   21篇
  2021年   45篇
  2020年   31篇
  2019年   28篇
  2018年   33篇
  2017年   40篇
  2016年   44篇
  2015年   27篇
  2014年   45篇
  2013年   94篇
  2012年   73篇
  2011年   95篇
  2010年   80篇
  2009年   57篇
  2008年   90篇
  2007年   66篇
  2006年   57篇
  2005年   59篇
  2004年   38篇
  2003年   30篇
  2002年   34篇
  2001年   21篇
  2000年   24篇
  1999年   103篇
  1998年   810篇
  1997年   419篇
  1996年   262篇
  1995年   161篇
  1994年   123篇
  1993年   164篇
  1992年   37篇
  1991年   38篇
  1990年   39篇
  1989年   34篇
  1988年   37篇
  1987年   46篇
  1986年   32篇
  1985年   24篇
  1984年   17篇
  1983年   23篇
  1982年   12篇
  1981年   18篇
  1980年   35篇
  1979年   11篇
  1978年   8篇
  1977年   38篇
  1976年   115篇
  1975年   10篇
  1973年   5篇
排序方式: 共有3794条查询结果,搜索用时 15 毫秒
61.
The efficacy of an early, intensive, supervised rehabilitation program to accelerate knee strength recovery in the first 3 weeks postmeniscectomy by arthroscopy was evaluated using a randomized controlled trial design. The maximal voluntary isokinetic strength of 31 men, randomly allocated to either a treatment (EXP) or a control (CTL) group, was measured twice by a blind rater: preoperatively (pretest) and 3 weeks postsurgery (posttest), using a computer-controlled Kin-Com dynamometer (Chattecx Corporation, Chattanooga, TN). Strength deficits of the operated leg at the pretest and posttest were established in percent of the values obtained for the sound leg at the pretest. In the interval between the surgery and the posttest, the patients of the EXP group (n = 15) received nine supervised treatments combined to home exercises whereas patients of the control group (n = 16) had no specific physiotherapy treatment but were given instructions in postsurgical management and prescribed exercises by the orthopedic surgeons. Patients of the EXP group had better knee extensor strength recovery than patients of the CTL group (ANCOVA, p < 0.001). The size of the strength difference (3 weeks postsurgery) between EXP and CTL subgroups (n = 8) matched according to preoperative deficits was as large as 26% and the residual deficits of the untreated patients were two to three times larger than those of the treated patients. The results of this study highlight the importance of instituting an early intensive and supervised rehabilitation program, especially for workers returning to a strenuous job requiring good knee extensor muscle function.  相似文献   
62.
OBJECTIVE: The aim of the study was to characterize n-3 and n-6 fatty acid delta5- and delta6-desaturase activities and their time course variations in human fetal liver between the 17th and 36th gestational week. STUDY DESIGN: Twenty-one biologic samples were obtained after legally approved medical abortion, according to French law. The desaturase activities were measured in the 21 liver samples by a radiochemical method by means of reverse-phase high-performance liquid chromatography. The fatty acid composition (percentage by weight) of liver phospholipids was assessed in 16 samples by gas-liquid chromatographic analysis. RESULTS: Both delta5- and delta6-desaturase activities were significantly expressed between the 17th and 36th gestational weeks. During the second trimester n-6 fatty acid delta5- and delta6-desaturase activities showed opposite patterns of variation; both then remained stable between the 25th and 36th weeks. Delta6-desaturation was higher in n-3 than n-6 fatty acids and peaked at the 18th gestational week. The percentages of linoleic and docosahexaenoic acids in liver microsomes were positively correlated with the gestation age (P < .01), whereas arachidonic acid remained stable. CONCLUSION: Significant n-3 and n-6 delta5- and delta6-desaturase activities are expressed in human fetal liver as early as the 17th gestational week and are stable throughout the third trimester. Their theoretic capacity evaluated from in vitro measurements appears lower than polyunsaturated fatty acid requirements and is not directly related to liver microsomal membrane fatty acid composition.  相似文献   
63.
This paper explores current applications for handheld devices and questions which of these make full use of the unique attributes of handheld devices in order to facilitate learning in a pedagogically sensible manner. In order to do so, the paper presents a functional framework which analyses handheld application in relation to their use as well as the pedagogical underpinning, if any, that informs their development. Our framework currently consists of seven distinct categories of application, which we term: administrative, referential, interactive, microworld, data collection, location aware and collaborative. We argue that three categories, namely data collection, location aware and collaborative, are particularly suited to learning with handheld devices when they are informed by collaborative, contextual and constructionist learning theories. Furthermore, we contend that applications of the type just outlined deserve further research since they are not attempting to replicate, or even augment, existing learning scenarios but rather they try to create new learning opportunities which would not be possible without (mobile) technology. Finally, the authors introduce a number of applications and learning scenarios that they have developed for handheld devices and explain their position within the framework.  相似文献   
64.
In order to obtain a bonded wood product from the tree, wood must be subjected to a series of processes. These include, among others, vital processes like drying of the green wood and machining of the surfaces of the dried wood in preparation for bonding. However, when wood is machined with dull blades, thermal degradation of the wood frequently occurs. The effects of kiln drying and thermal degradation of wood on the performance of the adhesive joints subsequently formed from such wood were investigated. Tapered double cantilever beam (TDCB) test specimens obtained from hard maple and phenol–resorcinol–formaldehyde (PRF) adhesives were used in the investigations. The enhancement of fracture energy due to the reduction in the moisture content of wood, the improved performance of adhesive joints due to rejointing of wood surfaces before bonding, and the reduction of adhesive joint strength resulting from thermal degradation of wood are presented and discussed.  相似文献   
65.
Strains of Bacteroides fragilis associated with diarrheal disease (enterotoxigenic B. fragilis) produce a 20-kDa zinc-dependent metalloprotease toxin (B. fragilis enterotoxin; BFT) that reversibly stimulates chloride secretion and alters tight junctional function in polarized intestinal epithelial cells. BFT alters cellular morphology and physiology most potently and rapidly when placed on the basolateral membrane of epithelial cells, suggesting that the cellular substrate for BFT may be present on this membrane. Herein, we demonstrate that BFT specifically cleaves within 1 min the extracellular domain of the zonula adherens protein, E-cadherin. Cleavage of E-cadherin by BFT is ATP-independent and essential to the morphologic and physiologic activity of BFT. However, the morphologic changes occurring in response to BFT are dependent on target-cell ATP. E-cadherin is shown here to be a cellular substrate for a bacterial toxin and represents the identification of a mechanism of action, cell-surface proteolytic activity, for a bacterial toxin.  相似文献   
66.
67.
The activities of levofloxacin and clarithromycin against 199 penicillin- and macrolide-susceptible and -resistant pneumococci were tested by agar and microdilution methods in air and by disk diffusion and E-test methods in air and CO2. For levofloxacin, >/=99. 0% of strains were susceptible at /=17 mm, regardless of incubation in air or CO2. Although zone sizes were smaller and E-test MICs were higher for clarithromycin in CO2 than those in air, category differences were minor, and susceptibility rates for clarithromycin were similar to those obtained by agar and microdilution in air (range, 76.9 to 80.9% by all methods). For clarithromycin, adjustment of breakpoints based upon distribution of results resulted in susceptibility rates which were similar by all methods (75.8 to 76.9% susceptible, 0 to 1.5% intermediate, 22.6 to 23.1% resistant). Minor discrepancies were obtained with levofloxacin for one strain (0.5%) by microdilution and two strains (1.0%) by disk diffusion in CO2. For clarithromycin, minor discrepancies were found in three strains (1.5%) by microdilution, seven strains (3.5%) by agar dilution, four strains (2.0%) by E-test in air, six strains (3.0%) by disk diffusion in air, and five strains (2.5%) by disk diffusion in CO2. Major discrepancies occurred with levofloxacin in one strain (0.5%) by microdilution but were not found with clarithromycin. Very major discrepancies were not seen with levofloxacin, but occurred with clarithromycin in five strains (2.5%) by microdilution, three strains (1.5%) by agar dilution, two strains (1.0%) by E-test in air, eight strains (4.0%) by disk diffusion in air, and one strain (0.5%) by disk diffusion in CO2.  相似文献   
68.
Splenomegaly is uncommon in myelodysplasia (MDS) and, although cytopenias may be severe, therapeutic splenectomy is rarely performed. We report the histologic, histochemical, and immunophenotypic findings of nine cases of surgical splenectomy and four postmortem spleens from MDS patients. Four histologic patterns were identified: one dominated by erythrophagocytosis, one characterized by red pulp plasmacytosis, one with extramedullary hematopoiesis as the only salient finding, and one with marked red pulp expansion caused by a monocytic proliferation. Wright-Giemsa and histochemical stains were performed on touch preparations in three cases and played a critical role in the precise subclassification of one MDS patient's hematologic disorder, which ultimately proved to be chronic myelomonocytic leukemia. Splenectomy led to sustained improvement of cytopenias in three cases, but did not eliminate transfusion dependence for the remaining patients. Three splenectomy cases exhibited clustered Leder-negative mononuclear elements: two of these patients experienced disease progression to refractory anemia with excess blasts in transformation or acute myelogenous leukemia during post-splenectomy follow-up, whereas none of the three splenectomy patients without clustered mononuclear elements did. We conclude that splenomegaly in MDS usually reflects the sequelae of dyspoiesis rather than evidence of a proliferative phase, that clustering of Leder-negative large cells may correlate with either a substantial monocytic component or, possibly, increased risk of disease progression, and that the spleen can provide diagnostic as well as prognostic information in MDS patients with splenomegaly.  相似文献   
69.
This article introduces the basic principles of reading electrocardiograms (ECGs) for nurses who are unfamiliar with reading them. For more experienced practitioners there are a number of useful articles and books (e.g. Hampton, 1992a, b) that will help further their knowledge. The ECG records cardiac electrical activity as a graph; interpretation is illustrated here by sinus rhythm. A single ECG lead (lead II) is used throughout this article. Atrial fibrillation is described to show a contrasting dysrhythmia. Specific nursing care is suggested for patients being monitored or having ECGs taken.  相似文献   
70.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号